A Frequency Support Strategy for Variable Speed Wind Turbines to Avoid Secondary Frequency Drop

With their increasing penetration into the power system, wind farms are required to participate in the provision of ancillary services such as frequency support (FS). One of the challenges associated with the participation of a wind turbine (WT) in FS is the potential for a secondary frequency drop (SFD) in the power system when the WT switches to recovery mode after frequency support. Several papers in the literature have attempted to solve or minimise the risk of SFD. However, these solutions introduce additional complexity such that the frequency support provided by the WT is drastically different from the emulation of inertia and droop based frequency support in conventional synchronous generators, thus challenging the predictability of the frequency dynamic behaviour of the power system, which is undesirable from a system planning and control point of view. To address this challenge, a novel FS strategy for variable speed wind turbines is proposed, which is capable of completely eliminating the risk of SFD while maintaining the predictability of the system frequency behaviour. The effectiveness of the proposed strategy is investigated and validated through EMT simulation tests on IEEE 9-bus power system model.